Document entry_point in cosmwasm-std instead of derive (backport #2245)

[mergify]

Closes #2186

Turns out we can document the re-export instead of the function definition. This also executes the doc tests as part of cosmwasm-std.

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This is an automatic backport of pull request #2245 done by Mergify.

[mergify]

Cherry-pick of e230048 has failed:

On branch mergify/bp/release/2.0/pr-2245
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You are currently cherry-picking commit e230048f1.
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Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   Cargo.lock
	both modified:   packages/derive/Cargo.toml
	both modified:   packages/derive/src/lib.rs
	both modified:   packages/std/src/lib.rs

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